In an intake pipe, a throttle valve is provided in order to control an intake air amount. Here, a problem arises in that noise may occur when the throttle valve is opened rapidly. A mechanism of occurrence of this noise is to be explained with reference to FIG. 10. FIG. 10 is a diagram for explaining a flow of air in the intake pipe in the beginning of the opening of the throttle valve. As shown in the figure, a throttle valve 300 is provided in an intake pipe 200. In general, the throttle valve 300 is configured to rotate around a rotating axis that is provided so as to extend in the horizontal direction. Therefore, in the beginning of the opening of the throttle valve 300, an air flow X1 through the upper side of the intake pipe 200 and an air flow X2 through the lower side thereof are created. It is considered that the noise occurs when the air flow X1 through the upper side and the air flow X2 through the lower side merge.
Conventionally, there is known a technique in which a flow-guiding net or a flow-guiding plate to guide an air flow is provided so that the occurrence of noise is suppressed (see Patent Literature 1). There is also known a technique in which a partition wall is provided so that the air flow through the upper side and the air flow through the lower side are prevented from merging (see Patent Literature 2).
However, in the case where the flow-guiding plate or the partition wall is provided, they create resistance when the air flows. Such resistance causes degradation in efficiency of air intake. On the other hand, in the case of the flow-guiding net, the resistance created during the air flow is not so large. However, in the case of the flow-guiding net according to the conventional art, although a flow-guiding function may be exhibited to a certain degree, it is difficult to sufficiently suppress the merging of the air flow X1 through the upper side and the air flow X2 through the lower side.